1. Field of the Invention
The present invention relates to a signal processing method of a global positioning system (GPS) receiver and a device thereof, and more particularly to a signal acquiring method of the GPS and a digital camera using the same.
2. Related Art
The positioning flow of GPS includes the following steps: data acquiring, signal acquisition, signal tracking, and positioning. Commonly used GPS receiver mainly receives L1 carrier wave and Coarse/Acquisition (C/A) code.
Frequencies of carrier waves used by GPS satellite signals are respectively L1 (1575.42 MHz) and L2 (1227.60 MHz). On L1, C/A code and precision (P) code are modulated, whereas on L2, only P code is modulated. GPS satellite signals respectively provide code information and navigation information. The code information is a Gold code generated by a pseudorandom noise (PRN) code generator, which is used to differentiate the satellites and calculating a virtual distance. The code information includes a C/A code with a chip rate of 1.023*106 (code/second), and with a code length of 1023. The P code has a chip rate of 10.23*106 (code/second).
The navigation information includes: data transmission time, satellite orbit data, satellite clock data, satellite distribution, and signal quality, etc. After a binary addition is performed on the code information and the navigation information, the carrier wave is modulated through a bi-phase shift keying (BPSK) modulation manner. Referring to FIG. 1, it is a schematic view of a BPSK modulation. The satellite sends the carrier wave after being modulated by BPSK to the ground.
Referring to FIG. 2, it is a schematic view of an architecture of a GPS receiver. The GPS receiver includes an antenna 210, a radio frequency (RF) front end 220, analog-to-digital converter 230, a reference oscillator 240, a frequency synthesizer 250, a digital signal processor 260, and a navigation processor 270. After the GPS receiver 200 receives satellite signals, a series of procedures are performed on the satellite signals through the RF front end 220, such as filtering, amplification, etc. Then, a wave mixing motion is performed on the signal output by the RF front end 220 with the carrier wave generated by the reference oscillator 240, and then the signal obtained after the wave mixing is input to the analog-to-digital converter 230, so as to obtain digitized satellite signals.
Then, an acquiring motion is performed on the digitized satellite signals, so as to obtain Doppler shift and recognition PRN code of the GPS carrier wave signal. Referring to FIG. 3, it is a schematic view of a satellite signal acquiring flow.
First, a Fourier transform is performed on the received satellite signals (Step S310). Next, a plurality of local codes 1(n) is generated according to an RF frequency (Step S320). Then, the Fourier transform is performed on each local code, to generate corresponding L(n) (Step S330). Then, 1(n) and L(n) obtained in Steps S320 and S330 are multiplexed with each other to output a corresponding first integral relevant signal Z(k) (Step S340). As for the operation process, please refer to the following equations, and it is assumed that the obtained satellite signal is a two-dimensional data array:
                                              ⁢                              z            ⁡                          (              n              )                                =                                    ∑                              m                =                0                                            N                -                1                                      ⁢                                          x                ⁡                                  (                  m                  )                                            ⁢                              y                ⁡                                  (                                      n                    +                    m                                    )                                                                                        Equation        ⁢                                  ⁢        1                                                          ⁢                              Z            ⁡                          (              k              )                                =                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                          ∑                                  m                  =                  0                                                  N                  -                  1                                            ⁢                                                x                  ⁡                                      (                    m                    )                                                  ⁢                                  y                  ⁡                                      (                                          n                      +                      m                                        )                                                  ⁢                                  ⅇ                                                            -                      j                                        ⁢                                                                                  ⁢                    2                    ⁢                    π                    ⁢                                                                                  ⁢                                          nk                      /                      N                                                                                                                              Equation        ⁢                                  ⁢        2                                                          ⁢                  =                                    ∑                              m                =                0                                            N                -                1                                      ⁢                                                            x                  ⁡                                      (                    m                    )                                                  ⁡                                  [                                                            ∑                                              n                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                  y                        ⁡                                                  (                                                      n                            +                            m                                                    )                                                                    ⁢                                              ⅇ                                                                              (                                                                                          -                                j                                                            ⁢                                                                                                                          ⁢                              2                              ⁢                                                              π                                ⁡                                                                  (                                                                      n                                    +                                    m                                                                    )                                                                                            ⁢                              k                                                        )                                                    /                          N                                                                                                      ]                                            ⁢                              ⅇ                                                      (                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                      π                      ⁢                                                                                          ⁢                      mk                                        )                                    /                  N                                                                                        Equation        ⁢                                  ⁢        3                                                          ⁢                  =                                                    Y                ⁡                                  (                  k                  )                                            ⁢                                                ∑                                      m                    =                    0                                                        N                    -                    1                                                  ⁢                                                      x                    ⁡                                          (                      m                      )                                                        ⁢                  e                  ⁢                                                                          ⁢                                      ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                      π                      ⁢                                                                                          ⁢                                              mk                        /                        N                                                                                                                  =                                          Y                ⁡                                  (                  k                  )                                            ⁢                                                X                                      -                    1                                                  ⁡                                  (                  k                  )                                                                                        Equation        ⁢                                  ⁢        4                                                      X                          -              1                                ⁡                      (            k            )                          =                                            ∑                              m                =                0                                            N                -                1                                      ⁢                                          x                ⁡                                  (                  m                  )                                            ⁢                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      mk                    /                    N                                                                                =                                                    [                                                      ∑                                          m                      =                      0                                                              N                      -                      1                                                        ⁢                                                            x                      ⁡                                              (                        m                        )                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  mk                          /                          N                                                                                                                    ]                            *                        =                                          X                *                            ⁡                              (                k                )                                                                        Equation        ⁢                                  ⁢        5            
in which X(k) is a conjugate function of X(k), and similarly, Y*(k) is a conjugate function of Y(k).|Z(k)|=|Y(k)X*(k)|=|Y*(k)X(k)|
Then, an inverse Fourier transform is performed on the first integral relevant value Z(k), thereby generating a second integral relevant value z(k). Finally, a maximum peak value of the second integral relevant value z(k) is obtained. If the received GPS signal includes a satellite signal with a corresponding PRN number, the peak value of the second integral relevant value is larger than the second integral relevant values of the other satellites. At this time, in the GPS receiver, the satellite signals with other PRN numbers are considered as noises. The GPS receiver can obtain the corresponding code information and carrier wave frequency according to the time point and frequency component of the peak value. If no peak value of the integral relevant value occurs during the acquiring process, the GPS receiver continuously searches for other satellites and satellite signals. Referring to FIG. 4, it is a schematic view of integral relevant values.
Since the GPS receiver performs the Fourier transform on the satellite signals, it costs time to calculate the real part and the imaginary part of the data, so only after the above operations are finished, the subsequent signal tracking and positioning processes can be executed one by one.